Datasheet

LT8640/LT8640-1
17
Rev.C
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APPLICATIONS INFORMATION
Spread Spectrum Mode
The LT8640/LT8640-1 features spread spectrum operation
to further reduce EMI/EMC emissions. To enable spread
spectrum operation, the SYNC/MODE pin should be tied
high to INTV
CC
(~3.4V) or an external supply of 3V to
4V. In this mode, triangular frequency modulation is
used to vary the switching frequency between the value
programmed by RT to approximately 20% higher than that
value. The modulation frequency is approximately 3kHz.
For example, when the LT8640/LT8640-1 is programmed
to 2MHz, the frequency will vary from 2MHz to 2.4MHz at
a 3kHz rate. When spread spectrum operation is selected,
Burst Mode operation is disabled, and the part will run in
either pulse-skipping mode (LT8640) or forced continuous
mode (LT8640-1).
Synchronization
To synchronize the LT8640/LT8640-1 oscillator to an
external frequency, connect a square wave (with 20%
to 80% duty cycle) to the SYNC/MODE pin. The square
wave amplitude should have valleys that are below 0.4V
and peaks above 1.5V (up to 6V).
The LT8640/LT8640-1 will not enter Burst Mode opera
-
tion at low output loads while synchronized to an external
clock, but instead will pulse-skip (LT8640) or run forced
continuous mode (LT8640-1) to maintain regulation. The
LT8640/LT8640-1 may be synchronized over a 200kHz to
3MHz range. The RT resistor should be chosen to set the
LT8640/LT8640-1 switching frequency equal to or below
the lowest synchronization input. For example, if the
synchronization signal will be 500kHz and higher, the RT
should be selected for 500kHz. The slope compensation is
set by the RT value, while the minimum slope compensation
required to avoid subharmonic oscillations is established
by the inductor size, input voltage and output voltage.
Since the synchronization frequency will not change the
slopes of the inductor current waveform, if the inductor
is large enough to avoid subharmonic oscillations at the
frequency set by RT, then the slope compensation will be
sufficient for all synchronization frequencies.
FB Resistor Network
The output voltage is programmed with a resistor divider
between the output and the FB pin. Choose the resistor
values according to:
R1= R2
V
OUT
0.970V
–1
⎛
⎝
⎜
⎞
⎠
⎟
(1)
Reference designators refer to the Block Diagram. 1%
resistors are recommended to maintain output voltage
accuracy.
If low input quiescent current and good light-load efficiency
are desired, use large resistor values for the FB resistor
divider. The current flowing in the divider acts as a load
current, and will increase the no-load input current to the
converter, which is approximately:
I
Q
= 1.7µA +
V
OUT
R1+R2
⎛
⎝
⎜
⎞
⎠
⎟
V
OUT
V
IN
⎛
⎝
⎜
⎞
⎠
⎟
1
n
⎛
⎝
⎜
⎞
⎠
⎟
(2)
where 1.7µA is the quiescent current of the LT8640/
LT8640-1 and the second term is the current in the feed
-
back divider reflected to the input of the buck operating
at its light load efficiency n. For a 3.3V application with
R1= 1M and R2 = 412k, the feedback divider draws 2.3µA.
With V
IN
= 12V and n = 80%, this adds 0.8µA to the 1.7µA
quiescent current resulting in 2.5µA no-load current from
the 12V supply. Note that this equation implies that the
no-load current is a function of V
IN
; this is plotted in the
Typical Performance Characteristics section.
When using large FB resistors, a 4.7pF to 22pF phase-lead
capacitor should be connected from V
OUT
to FB.
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